Surface acoustic wave filter

ABSTRACT

It is configured by forming first IDT ( 202 ) and second IDT ( 203 ) on piezoelectric substrate ( 201 ), and first IDT ( 202 ) is arranged between one terminal ( 204 ) of input/output terminals and other terminal ( 205 ) of the input/output terminals, i.e., serially to a signal path, and second IDT ( 203 ) is arranged in parallel to a signal path from a portion between one terminal ( 204 ) of the input/output terminals and first IDT ( 202 ). First IDT ( 202 ) and second IDT ( 203 ) are arranged in proximity to each other on the same propagation path of surface acoustic waves which are excited by respective resonators.

TECHNICAL FIELD

This invention relates to a surface acoustic wave filter which is usedfor a communication device etc. such as a portable telephone.

BACKGROUND ART

In recent years, along with development of mobile communication,realization of high performance and miniaturization of devices to beused have been desired. As a filter for a mobile communication deviceamong those devices, conventionally, a surface acoustic wave filter(hereinafter, called as a SAW filter) has been used widely. At thepresent day, as a SAW filter for an RF (Radio Frequency) stage, alongitudinal mode type and a ladder mode are mainly used. In particular,as to the ladder type SAW filter, it is possible to realize a low lossas compared with the longitudinal mode SAW filter. The ladder type SAWfilter is of such a configuration that a plurality of surface acousticwave resonators (hereinafter, called as SAW resonators) are connected ina ladder mode, and configured by SAW resonators operating as a serialarm and SAW resonators operating as a parallel arm.

As prior art relating to the ladder type SAW filter, JP-A-9-270663publication discloses a method of arranging SAW resonators operating asa serial arm and SAW resonators operating as a parallel arm, withisolating them by a certain level of a distance. FIG. 20 is a blockdiagram of a SAW filter configured in this manner and disclosed in theabove-mentioned first prior art document. This SAW filter is configuredby SAW resonators 902 as a serial arm, SAW resonators 903 as a parallelarm which are formed on piezoelectric substrate 901 and signal lines 904connected to these SAW resonators 902, 903. These SAW resonators 902,903 are arranged in such a manner that respective overlap lengths ofcomb-shaped electrodes configuring these SAW resonators 902, 903 overlapin a propagation direction of leaky surface acoustic wave. In this case,if it is set up in such a manner that a distance of a gap between SAWresonator 902 as the serial arm and SAW resonator 903 as the parallelarm becomes 10 times more of a wavelength of the leaky surface acousticwave, it is possible to prevent interference of leaky surface acousticwave of these SAW resonators 902, 903.

As another prior art relating to the ladder type SAW filter,JP-A-9-232908 publication discloses a method of arranging a slit platebetween SAW resonators operating as a serial arm and SAW resonatorsoperating as a parallel arm. FIG. 21 is a block diagram of a SAW filterof the above-mentioned second prior art document, which was configuredin this manner. This SAW filter is configured by SAW resonators 1002,1005 of a serial arm, SAW resonators 1003, 1006 of a parallel arm andslit plates 1004, 1007, which are formed on piezoelectric substrate1001. Slit plate 1004 is disposed between SAW resonator 1002 of theserial arm and SAW resonator 1003 of the parallel arm, to block offsurface acoustic waves leaking from respective SAW resonators 1002,1003. In the same manner, slit plate 1007 is disposed between SAWresonator 1005 of the serial arm and SAW resonator 1006 of the parallelarm, to block off surface acoustic waves leaking from respective SAWresonators 1005, 1006.

As another prior art relating to the ladder type SAW filter,JP-A-2000-201052 publication discloses a method of arranging in such amanner that propagation paths of surface acoustic wave in SAW resonatorsoperating as a serial arm and SAW resonators operating as a parallel armdo not overlap. FIG. 22 is a block diagram of a SAW filter of theabove-mentioned third prior art document, which was configured in thismanner. This SAW filter is configured by SAW resonators 1102, 1103 of aserial arm, and SAW resonator 1004 of a parallel arm, which are formedon piezoelectric substrate 1101. In this configuration, it is formed insuch a manner that a surface acoustic wave propagation path of SAWresonator 1104 of the parallel arm, which is sandwiched by two SAWresonators 1102, 1103 of the serial arm, is located between surfaceacoustic wave propagation paths of these SAW resonators 1102, 1103 ofthe serial arm. By this means, surface acoustic waves of respective SAWresonators of the serial arm and the parallel arm do not interfere witheach other, and a good filter characteristic is obtained.

However, in the above-mentioned conventional disclosure examples, SAWresonators of a serial arm and SAW resonators of a parallel arm arearranged in an isolated manner by a certain level of a distance. On thisaccount, there was such a problem that a size of a SAW filter becomeslarge. These SAW filters disclose a technique of eliminating aninterference of surface acoustic surface waves of SAW resonators of aserial arm and SAW resonators of a parallel arm, but do not disclose away of thinking for utilizing this at all.

The present invention aims to provide a SAW filter which is of a smallsize and enables a low loss, by a SAW filter with such a newconfiguration that a plurality of SAW resonators are arranged inproximity to each other on the same propagation path, against theabove-mentioned problem.

DISCLOSURE OF THE INVENTION

In order to accomplish the above-mentioned aimed object, a SAW filter ofthe present invention is comprised of a piezoelectric substrate, and atleast two inter-digital transducers (hereinafter, called as IDTs)disposed in proximity to each other on the same surface acoustic wavepropagation path on this piezoelectric substrate, and at least one ofthe IDTs is a first IDT connected serially to a signal path and at leastone is a second IDT connected between the signal path and a ground, andthe first IDT and the second IDT are different in resonance frequency,and the first IDT and the second IDT are formed by such a configurationthat electrode fingers of comb-shaped electrodes configuring IDT arearranged almost continuously.

According to this configuration, it is possible to obtain a SAW filterof a small size and a low loss, over having an attenuationcharacteristic equivalent to that of a so-called L type configuration ofa ladder mode surface acoustic wave filter which is used conventionally.Such a configuration that electrode fingers are arranged almostcontinuously means that they are arranged in such a manner that adifference of a gap between adjacent electrode finger and electrodefinger falls in an error of approximately 5% or less. By arranging inthis manner, it is possible to prevent occurrence of loss due to bulkwave conversion at a boundary between IDT and IDT. Alternately, in caseof arranging a strip line electrode between IDTs as described later, itis possible to prevent occurrence of loss due to bulk wave conversion ata boundary between IDT and the strip line electrode. It is also possibleto utilize a second IDT from a view point of a first IDT, or the firstIDT from a view point of the second IDT, as a reflector, respectively.

In the above-mentioned configuration, it is all right even if the firstIDT and the second IDT are arranged in such a manner that respectivesurface acoustic waves are not negated with each other. In this case, itis also all right even if the first IDT and the second IDT areconfigured to be turned into reversed phases each other.

According to this configuration, it is possible to realize such aconfiguration that mutual surface acoustic waves are not denied, andtherefore, it is possible to realize a SAW filter of a small size andsmall loss, by disposing a plurality of IDTs with different resonancefrequencies in a propagation direction of surface acoustic waves.

In the above-mentioned configuration, it is all right even if it isconfigured in such a manner that resonance frequencies of the first IDTand the second IDT are set up to frequencies necessary for obtaining apreset filter characteristic. In that case, it is all right even ifresonance frequency of the first IDT is roughly matched withanti-resonance frequency of the second IDT. By this configuration, it ispossible to easily obtain a targeted filter characteristic.

In the above-mentioned configuration, it is all right even if areflector electrode is disposed at the outermost side of IDT includingthe first IDT and the second IDT. According to this configuration, it ispossible to effectively close surface acoustic waves in IDT, andtherefore, it is possible to realize a SAW filter by which loss is muchmore reduced.

In the above-mentioned configuration, it is all right even if a stripline electrode is disposed between the first IDT and the second IDT, andelectrode fingers of comb-shaped electrodes which configure the firstIDT and the second IDT, and electrode fingers which configure this stripline electrode are arranged almost continuously. In this case, it isalso all right even if a pitch of electrode fingers of this strip lineelectrode is set up between a pitch of electrode fingers of the firstIDT and a pitch of electrode fingers of the second IDT. According tothis configuration, it is possible to effectively close over surfaceacoustic waves which are excited from respective IDTs in IDTs, andtherefore, it is possible to reduce loss.

In the above-mentioned configuration, it is all right even if it isconfigured in such a manner that a pitch of plural electrode fingers,which are arranged in a boundary area of the first IDT and the secondIDT, is differentiate from a pitch of electrode fingers which arearranged in respective center areas. In this case, it is also all righteven if weighting method is applied to at least one of IDTs whichconfigure a SAW filter. As this weighting method, apodized weighting, orwithdrawal weighting may be applied.

According to this configuration, it is possible to easily realize a SAWfilter by which an attenuation is large and loss is small, by adjustingan attenuation and loss in tune with a design, in a necessitatedfrequency domain.

In the above-mentioned configuration, it is all right even if IDT isconfigured so as to include a dummy electrode. By arranging andoptimizing dummy electrodes respectively, it is possible to realize aSAW filter with much lower loss.

In the above-mentioned configuration, it is all right even if it isconfigured in such a manner that a third IDT, which is connected betweena signal path and a ground, is arranged in proximity to each other tothe first IDT, on an opposite side to such a side that the second IDT isarranged in proximity to each other. In this case, it is all right evenif resonance frequency of the third IDT is differentiated from resonancefrequency of the first IDT.

In the above-mentioned configuration, it is all right even if a fourthIDT, which is connected serially to a signal path, is arranged inproximity to each other to the second IDT, on an opposite side to such aside that the first IDT is arranged in proximity to each other. In thiscase, it is all right even if resonance frequency of the fourth IDT isdifferentiated from resonance frequency of the second IDT.

A SAW filter of the present invention is formed by such a configurationthat a SAW filter with the above-mentioned configuration is used as oneSAW element, and these SAW elements are connected in multiple stages.According to this configuration, it is possible to easily obtain a SAWfilter having a targeted characteristic, and it is also possible toenlarge a freedom degree of design.

According to the present invention, by arranging a plurality of SAWresonators in proximity to each other to the same propagation path, itis possible to realize a small size shape even in case of a SAW filterin which a high attenuation characteristic is demanded and a lot ofresonators are required. In case that these SAW filters are connected inmultiple stages as one element, it is possible to simplify a wiringpattern, and therefore, it is possible to reduce electric resistance ofthe wiring pattern, and it is also possible to realize a SAW filter bywhich loss is reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view showing a configuration of a SAW filter whichrelates to a first embodiment of the present invention.

FIG. 2 is a pattern diagram for explaining a reversed phaseconfiguration in the same embodiment.

FIG. 3 is a plan view showing a SAW filter of a first modified examplein the same embodiment.

FIG. 4 is a plan view showing a SAW filter of a second modified examplein the same embodiment.

FIG. 5 is a plan view of a SAW filter which is formed by such aconfiguration that a SAW filter of an one stage L type configuration,which relates to the same embodiment, is used as a basic SAW element,and these SAW elements are cascade-connected in two stages.

FIG. 6 is a plan view explaining such a configuration that the SAWfilter shown in FIG. 1 is used as a basis, and a gradation area isdisposed.

FIG. 7 is a view showing by enlarging a configuration of electrodefingers in a boundary area of the SAW filter shown in FIG. 6.

FIG. 8 is a graph showing a result of measuring the characteristic of aSAW filter of a practical example 1.

FIG. 9 is a graph showing a result of measuring the characteristic of aSAW filter of a practical example 2.

FIG. 10 is a graph showing a result of measuring the characteristic of aSAW filter of a practical example 3.

FIG. 11 is a graph showing a result of measuring the characteristic of aSAW filter of a comparative example 1.

FIG. 12 is a graph showing a result of measuring the characteristic of aSAW filter of a practical example 4.

FIG. 13 is a graph showing a result of measuring the characteristic of aSAW filter of a comparative example 2.

FIG. 14 is a plan view of a SAW filter of a practical example 5 which isconfigured in such a manner that the SAW filter shown in FIG. 3 is usedas a basic SAW element, and these SAW elements are cascade-connected infour stages.

FIG. 15 is a graph showing a result of measuring the characteristic asto a SAW filter of the practical example 5.

FIG. 16 is a plan view showing a configuration of a SAW filter of acomparative example 3, which is produced for comparison and in which SAWresonators formed by a conventional ladder configuration arecascade-connected in multiple stages.

FIG. 17 is a graph showing a result of measuring the characteristic asto the SAW filter of the comparative example 3.

FIG. 18 is a plan view showing a configuration of a SAW filter whichrelates to a second embodiment of the present invention.

FIG. 19 is a plan view showing a configuration of a SAW filter whichrelates to a third embodiment of the present invention.

FIG. 20 is a block diagram of a conventional SAW filter disclosed in afirst prior art document.

FIG. 21 is a block diagram of a conventional SAW filter disclosed in asecond prior art document.

FIG. 22 is a block diagram of a conventional SAW filter disclosed in athird prior art document.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be explained indetail by use of drawings. Since the same reference numerals and signsare given to the same elements, there is such a case that explanationswill be omitted. Drawings shown below are pattern views, and the numberof electrode fingers and a pitch are not described accurately.

First Embodiment

FIG. 1 is a plan view showing a configuration of a SAW filter whichrelates to a first embodiment. In general, a SAW filter is packaged byceramic, resin etc., and used, but FIG. 1 shows only a configuration ona piezoelectric substrate 201. As shown in FIG. 1, in case of a SAWfilter of this embodiment, it is composed by such a configuration thatfirst IDT 202 and second IDT 203 are arranged on a surface ofpiezoelectric substrate 201 which is formed by lithium tantalate(LiTaO₃) of 39° Y-cut X propagation in such a manner that they are onthe same propagation path, reflector electrodes 207 are disposed onrespective end portions.

First IDT 202 is arranged between one terminal 204 and the otherterminal 205 of input/output terminals, i.e., serially to a signal path,and can carry out an operation which is equivalent to that of SAWresonators of a serial arm. Second IDT 203 is arranged in parallel tothe signal path from a portion between one terminal 204 of theinput/output terminals and first IDT 202. This second IDT 203 isconnected to one terminal 204 and the first IDT by wiring electrode 206.By the suchlike arrangement and connection configuration, second IDT 203can carry out an operation which is equivalent to that of SAW resonatorsof a parallel arm.

As to first IDT 202 and second IDT 203, electrode fingers of comb-shapedelectrode, which configure respective IDTs, are arranged almostcontinuously, on the same propagation path of surface acoustic waveswhich are excited by respective SAW resonators. At this time, first IDT202 and second IDT 203 are configured in such a manner that surfaceacoustic waves, which are excited by respective SAW resonators, are notnegated with each other. Reflector electrodes 207 are arranged on anopposite side to an adjacent side of first IDT 202 and second IDT 203.The SAW filter of first embodiment is formed by adopting the suchlikeconfiguration.

Wiring electrode 206 and one terminal 204 and other terminal 205 of thecomb-shaped electrode which configures first IDT 202 and other terminal205 are respectively connected by a wire lead etc. The other of thecomb-shaped electrode, which configures second IDT 203, is connected toan earth terminal by a wire lead etc. in the same manner, and isgrounded.

An electrode finger pitch of the comb-shaped electrode which configuresfirst IDT 202 is smaller than an electrode finger pitch of thecomb-shaped electrode which configures second IDT 203, and respectivepitches are set up so as to realize a filter characteristic based on adesign value. An electrode finger pitch of reflector electrode 207 isset up so as to become an intermediate value between an electrode fingerpitch of first IDT 202 and an electrode finger pitch of second IDT 203.

By arranging first IDT 202 and second IDT 203 in proximity to each otherto close over them on one propagation path, as to respective SAWresonators, it becomes equivalent to such a matter that its resonatorlength is lengthened in substance, and it is possible to improve a SAWresonator characteristic. In consequence, as a SAW filter, it ispossible to obtain a band-pass type and small loss characteristic. Bysuch a matter that it is possible to arrange first IDT 202 and secondIDT 203 in proximity to each other and to shorten a wiring electrode,miniaturization of a SAW filter can be realized. The above-describedconfiguration also becomes a basic configuration unit of an L type SAWfilter.

As described above, a SAW filter of the present invention can realize asmall size and small loss filter characteristic, by arranging aplurality of SAW resonators in proximity to each other on the samepropagation path.

As a method of changing resonance frequencies of first IDT 202 andsecond IDT 203, there are a method of changing a width of electrodefingers, a method of changing a pitch of electrode fingers, and so on.However, judging from a view point of such a matter that it is possibleto enlarge a design freedom degree of a SAW filter and of easiness of aprocess, the method of changing an electrode finger pitch is desirable.

Generally speaking, in case of configuring SAW resonators, respectivepitches are determined in such a manner that peak frequency of an IDTcharacteristic is matched with center frequency of a reflectioncharacteristic of a reflector electrode. However, since the reflectioncharacteristic is flat in a relatively wide frequency area, a resonancecharacteristic is obtained if it is in this flat area, without beingaccompanied with large deterioration of a characteristic. Therefore, itis all right if a reflection characteristic falls in a flat area in peakfrequency of all IDT characteristics, and it is desirable to make apitch of reflector electrodes larger than a pitch of IDTs having aminimum pitch, and smaller than a pitch of IDTs having a maximum pitch.By arranging IDTs in such a manner that respective surface acousticwaves are not negated, it is possible to avoid mutual interference.

In case of arranging first IDT 202 and second IDT 203 in such a mannerthat surface acoustic waves are not negated, it is desirable toconfigure in such a manner that first IDT 202 and second IDT 203 to beturned into reversed phases each other.

The reversed phase in the present invention means a configuration asshown in FIG. 2. FIG. 2 is a pattern diagram for explaining the reversedphase, and shows it by enlarging a boundary area portion of first IDT 22and second IDT 23. In this figure, strip line electrode 31 is disposedbetween first IDT 22 and second IDT 23. Surface acoustic wave 25, whichis excited from first IDT 22 and second IDT 23, is also shownschematically. In case of the suchlike configuration, as shown in thefigure, peaks and troughs of surface acoustic wave 25 are formed byelectrode finger 221 which is the closest to second IDT 23, in first IDT22, and electrode finger 231 which is the closest to first IDT 22, insecond IDT 23, and one comb-shaped electrodes, which configure first IDT22 and second IDT 23, are connected in common. The reversed phase in thepresent invention means the suchlike arrangement configuration. In orderto realizing the suchlike reversed phase, strip line electrode 31 shownin the figure is not indispensable, and the reversed phase configurationis possible without disposing strip line electrode 31.

In the SAW filter of this embodiment, reflector electrode 207 isarranged, but the present invention is not limited to this. That is, incase of such a configuration that containment of surface acoustic wavescan be realized more sufficiently than in another method, it is allright even if reflector electrode 207 is not particularly disposed.

FIG. 3 is a plan view showing a SAW filter of a first modified examplein this embodiment. Also in the SAW filter of this modified example,only a configuration on piezoelectric substrate 201 is shown in the samemanner as in FIG. 1. This SAW filter of the first modified example ischaracterized in that roughly several pieces of strip line electrodes301 are arranged between first IDT 202 and second IDT 203. It ispreferable that its electrode finger pitch is set up so as to become anintermediate value between an electrode finger pitch of first IDT 202and an electrode finger pitch of second IDT 203. A pitch of electrodefingers of strip line electrode 301 is set up so as to become theabove-mentioned intermediate value, but there is no need to make allpitches of electrode fingers constant, and it is all right even if it ischanged in a stepwise fashion and it is also all right even if it ismade as gradation in which it is changed continuously. This strip lineelectrode 301 is operated as a reflector electrode.

FIG. 4 is a plan view showing a SAW filter of a second modified examplein this embodiment. Also in the SAW filter of this modified example,only a configuration on piezoelectric substrate 201 is shown in the samemanner as in FIG. 1. This SAW filter of the second modified example ischaracterized in that dummy electrodes 401, 402 shown in the figure aredisposed on first IDT 404 and second IDT 405, respectively, and othersare the same as the SAW filter configuration shown in FIG. 1. Byarranging these dummy electrodes 401, 402 respectively to carry outoptimization, it is possible to realize a SAW filter with much lowerloss.

In the above-descried embodiment, a one stage, L type configuration SAWfilter was explained, but the present invention is not limited to this.For example, it is all right even if a multiple stage configuration SAWfilter shown in FIG. 5 is realized.

FIG. 5 is a plan view of a SAW filter which is formed by such aconfiguration that the one stage, L type configuration SAW filter shownin FIG. 1 is used as a basic SAW element and these SAW elements arecascade-connected in two stages. First SAW element 501 is formed by sucha configuration that first IDT 2021 and second IDT 2031 are arranged ona surface of piezoelectric substrate 201 so as to be located on the samepropagation path, and reflector electrodes 2071 are disposed onrespective end portions. Second SAW element 502 is formed by such aconfiguration that first IDT 2022 and second IDT 2032 are arranged on asurface of piezoelectric substrate 201 so as to be located on the samepropagation path and reflector electrodes 2072 are disposed onrespective end portions, in the same manner.

One of comb-shaped electrodes which configure first IDT 2021 isconnected to one terminal 204 of input/output terminals, and the otherof the comb-shaped electrodes is connected to wiring electrode 206. Oneof comb-shaped electrodes which configure second IDT 2031 is connectedto earth, and the other is connected to wiring electrode 206.

One of comb-shaped electrodes which configure first IDT 2022 isconnected to one terminal 205 of input/output terminals, and the otherof the comb-shaped electrodes is connected to wiring electrode 206. Oneof comb-shaped electrodes which configure second IDT 2032 is connectedto earth, and the other is connected to wiring electrode 206.

By adopting the suchlike configuration, a pattern shape including IDTsand reflector electrodes formed on piezoelectric substrate 201 becomesalmost a rectangular shape as a whole. Therefore, a space factor of apattern on a chip is improved. Inconsequence, miniaturization of a chipas a SAW filter becomes possible. Connection between first SAW element501 and second SAW element 502 is realized only by wiring electrode 206,and since a very simple configuration is realized, realization of a lowloss can be carried out.

Hereinafter, a concrete configuration of a SAW filter which relates tothis embodiment and a result of its characteristic which was obtained bysimulation will be explained.

FIG. 6 is a plan view explaining such a configuration that the SAWfilter shown in FIG. 1 is used as a basis and a gradation area isdisposed. This SAW filter is formed by such a configuration that firstIDT 202 and second IDT 203 have different resonance frequencies, and asto first IDT 202 and second IDT 203, electrode fingers of comb-shapedelectrodes which configure IDTs are arranged almost continuously in sucha manner that respective surface acoustic waves are not negated.Hereinafter, this SAW filter is called as a SAW filter of a practicalexample 1.

Concretely speaking, this SAW filter of the practical example 1 ischaracterized by such a configuration that a pitch of plural electrodefingers of gradation areas 2026, 2036, which are boundary areas of firstIDT 202 and second IDT 203, is differentiated from a pitch of electrodefingers of equal pitch areas 2025, 2035 at respective center areas,i.e., gradation is disposed. FIG. 7 is a view showing by enlarging aconfiguration of electrode fingers in this boundary area. FIG. 7 is aschematic view, and does not express the number of electrode fingersetc. accurately. TABLE 1 shows the number of electrode fingers andpitches of this SAW filter.

As understood from TABLE 1 and FIG. 7, 215 pieces of electrode fingersare disposed in equal pitch area 2025 of first IDT 202, and a pitch inthis area is 2.341 μm. Gradation area 2026 is disposed in a region froman end portion of this equal pitch area 2025 up to a boundary portion ofsecond IDT 203. The number of electrode fingers in gradation area 2026is 10 pieces, and its pitch is changed continuously from 2.341 μm as apitch of end portions in the equal pitch area up to 2.411 μm as a pitchof the boundary portion of second IDT 203.

Gradation area 2036 of second IDT 203 is changed continuously from 2.411μm as a pitch of end portions in gradation area 2026 of first IDT 202 upto 2.429 μm as a pitch of equal pitch area 2035. The number of electrodefingers in this gradation area 2036 is 10 pieces, and the number ofelectrode fingers in equal pitch area 2035 is 135 pieces.

The number of electrode fingers of reflector electrode 207 which isadjacent to first IDT 202 is 35 pieces, and its pitch is set to 2.404μm. The number of electrode fingers of reflector electrode 207 which isadjacent to second IDT 203 is 35 pieces, and its pitch is set to 2.418μm. Piezoelectric substrate 201 is formed by lithium tantalate (LiTaO₃)of 39° Y-cut X propagation, and an electrode film thickness is set toapproximately 0.4 μm. A characteristic of the SAW filter of thepractical example 1 which is formed by the above-mentioned configurationis obtained, and its result is shown in FIG. 8. As understood from FIG.8, in the SAW filter of the practical example 1, losses at 824 MHz and849 MHz were 0.65 dB, 0.28 dB. TABLE 1 Number of Electrode Fingers(piece) Pitch (μm) Reflector Electrode 207 35 2.404 First Equal Pitch215 2.341 IDT 202 Area 2025 Gradation 10 2.411 Area 2026 (right endportion of first IDT 202) Second Gradation 10 2.411 IDT 203 Area 2036(left end portion of second IDT 203) Equal Pitch 135 2.429 Area 2035Reflector Electrode 207 35 2.418*An overlap length is 115 μm, and η is 0.52, and made constant.

Next, in the same manner as the SAW filter of the practical example 1,manufactured was such a SAW filter that thinned-out apodization wasadded further to such a configuration that a gradation area is disposed.This is called as a SAW filter of a practical example 2. The SAW filterof the practical example 2 is the same as the SAW filter of thepractical example 1 as to its configuration, but the number of electrodefingers and a pith are different.

TABLE 2 shows the number of electrode fingers and pitches of the SAWfilter of the practical example 2. AS to things other than aconfiguration shown in TABLE 2, they are the same as those of the SAWfilter of the practical example 1, and therefore, explanations will beomitted.

A characteristic of the SAW filter of the practical example 2, which isformed by the above-mentioned configuration, is obtained, and its resultis shown in FIG. 9. As understood from FIG. 9, in the SAW filter of thepractical example 2, losses at 824 MHz and 849 MHz were 0.52 dB, 0.29dB. TABLE 2 Number of Electrode Fingers (piece) Pitch (μm) ReflectorElectrode 207 35 2.383 First Equal Pitch 215 2.321 IDT 202 Area 2025Gradation 10 2.390 Area 2026 (right end portion of first IDT 202) SecondGradation 10 2.390 IDT 203 Area 2036 (left end portion of second IDT203) Equal Pitch 135 2.408 Area 2035 Reflector Electrode 207 35 2.397*An overlap length is 115 μm, and η is 0.52, and made constant.

In the same manner as the SAW filter of the practical example 1,manufactured was such a SAW filter that weighting method was addedfurther to such a configuration that a gradation area is disposed. Thisis called as a SAW filter of a practical example 3. As to the weightingmethod, its optimum value was set up over evaluating a characteristic bysimulation. A characteristic of the SAW filter of the practical example3 which was manufactured in this manner was obtained, and its result isshown in FIG. 10. As understood from FIG. 10, in the SAW filter of thepractical example 3, losses at 824 MHz and 849 MHz were 0.56 dB, 0.28dB.

In order to compare with a SAW filter of the present invention,manufactured was a SAW filter which is formed by a conventional laddermode configuration. This is called as a SAW filter of a comparativeexample 1. A characteristic of the SAW filter of this comparativeexample 1 was measured, and its result is shown in FIG. 11. Asunderstood from FIG. 11, in the SAW filter of the comparative example 1,losses at 824 MHz and 849 MHz were 0.65 dB, 0.28 dB.

These characteristics are shown collectively in TABLE 3. As understoodfrom TABLE 3, it could be confirmed that the SAW filters of thepractical example 2 and the practical example 3 can obtain morefavorable characteristic than that of the SAW filter of the comparativeexample 1, except for the SAW filter of the practical example 1. In caseof the SAW filter of the practical example 1, its characteristic was thesame as that of the SAW filter of the comparative example 1, but it ischaracterized on such a point that it is possible to miniaturize it muchmore than the SAW filter of the comparative example 1. TABLE 3 SAWfilter SAW filter SAW filter SAW filter of of Practical of Practical ofPractical Comparative Example 1 Example 2 Example 3 Example 1 824 MHz0.65 dB 0.52 dB 0.56 dB 0.65 dB 894 MHz 0.28 dB 0.29 dB 0.28 dB 0.28 dB

Next, manufactured was a SAW filter which was configured in such amanner that a strip line electrode is disposed between a first IDT and asecond IDT, and electrode fingers of comb-shaped electrodes whichconfigure the first IDT and the second IDT, and electrode fingers whichconfigure the strip line electrode or a reflector electrode are arrangedso as to become almost continuously. This is based on the SAW filtershown in FIG. 3, and the number of electrode fingers and pitches of themanufactured SAW filter are shown in TABLE 4. This is called as a SAWfilter of a practical example 4.

A characteristic of the SAW filter of the practical example 4manufactured based on TABLE 4 was obtained, and its result is shown inFIG. 12. As understood from FIG. 12, in the SAW filter of the practicalexample 4, losses at 824 MHz and 849 MHz were 0.70 dB, 0.25 dB. TABLE 4Number of Electrode Fingers (piece) Pitch (μm) Reflector Electrode 20735 2.404 First IDT 202 225 2.341 Strip Line Electrode 301 35 2.411Second IDT 203 145 2.429 Reflector Electrode 207 35 2.418

In order to compare with the SAW filter of the practical example 4, alsomanufactured was a SAW filter of a conventional ladder modeconfiguration. This is called as a SAW filter of a comparative example2. A characteristic of the SAW filter of this comparative example 2 wasmeasured, and its result is shown in FIG. 13. As understood from FIG.13, in the SAW filter of the comparative example 2, losses at 824 MHzand 849 MHz were 0.70 dB, 0.25 dB.

In case of the SAW filter of the practical example 4, a characteristicitself was of the same result as that of a conventional ladder modeconfiguration, but it is characterized on such a point thatminiaturization is possible.

Next, manufactured was a SAW filter in which the SAW filter shown inFIG. 3 is used as a basic SAW element and these things arecascade-connected in multiple stages, and its characteristic wasobtained, and its result will be described.

FIG. 14 is a plan view of a SAW filter which is configured in such amanner that the SAW filter shown in FIG. 3 is used as a basic SAWelement and these SAW elements are cascade-connected in 4 stages. Thisis called as a SAW filter of a practical example 5. Piezoelectricsubstrate 201 is formed by use of lithium tantalate (LiTaO₃) of 39°Y-cut X propagation, and an electrode film thickness is set toapproximately 0.4 μm. The number of electrode fingers and pitches ofrespective IDTs and reflector electrodes from a first stage up to afourth stage are shown in TABLE 5.

A first stage of this SAW filter of the practical example 5 isconfigured by first IDT 502, second IDT 503, reflector electrodes 504,505 on both sides, and strip line electrode 506 at a center portion. Asecond stage is configured by first IDT 512, second IDT 513, reflectorelectrodes 514, 515 on both sides, and strip line electrode 516 at acenter portion. A third stage is configured by first IDT 522, second IDT523, reflector electrodes 524, 525 on both sides, and strip lineelectrode 526 at a center portion. A final fourth stage is configured byfirst IDT 532, and reflector electrodes 534, 535 which are arranged onboth sides thereof. TABLE 5 First Stage Reflector First Strip LineSecond Reflector Electrode IDT Electrode IDT Electrode 504 502 506 503505 Overlap lengths — 115 — 115 — (μm) Number of 35 225 23 145 35Electrode Fingers (piece) Electrode Finger 2.404 2.341 2.411 2.429 2.418Pitch (μm) Second Stage Reflector First Strip Line Second ReflectorElectrode IDT Electrode IDT Electrode 514 512 516 513 515 Overlaplengths — 55 — 55 — (μm) Number of 35 157 23 181 35 Electrode Fingers(piece) Electrode Finger 2.404 2.334 2.411 2.427 2.418 Pitch (μm) ThirdStage Reflector First Strip Line Second Reflector Electrode IDTElectrode IDT Electrode 524 522 526 523 525 Overlap lengths — 75 — 75 —(μm) Number of 35 201 23 153 35 Electrode Fingers (piece) ElectrodeFinger 2.415 2.434 2.406 2.309 2.392 Pitch (μm) Fourth Stage ReflectorReflector Electrode First IDT Electrode 534 532 535 Overlap lengths — 40— (μm) Number of 35 235 35 Electrode Fingers (piece) Electrode Finger2.411 2.334 2.411 Pitch (μm)

These IDTs are connected by wiring electrodes 540, 541, 542.

One of comb-shaped electrodes, which configure first IDT 502 at a firststage, is connected to one terminal 204 of input/output terminals, andthe other of the comb-shaped electrodes, which configure first IDT 532at a fourth stage, is connected to other terminal 205 of input/outputterminals. Strip line electrodes 506, 516, 526 are operated as reflectorelectrodes, respectively.

FIG. 15 is a graph showing a result of measuring the characteristic asto a SAW filter of the practical example 5 with the suchlikeconfiguration. AS understood from FIG. 15, in the SAW filter of thepractical example 5, losses at 824 MHz and 849 MHz were 0.92 dB, 1.29dB.

In order to compare with this SAW filter, manufactured was a SAW filterof such a configuration that SAW resonators, which are formed by aconventional ladder mode configuration, are cascade-connected inmultiple stages. FIG. 16 is a plan view showing a configuration of thisSAW filter. This SAW filter is configured by SAW resonators 600 of aserial arm formed on piezoelectric substrate 650, SAW resonators 610 ofa parallel arm, and signal line 640 connected to these SAW resonators600, 610. SAW resonators 600 of a serial arm are connected to oneterminal 620 and other terminal 630 of input/output terminals,respectively. The number of electrode fingers, a pitch, a overlaplengths etc. are set up to be matched with a pass band of the SAW filterof the practical example 5.

A SAW filter of the suchlike configuration is hereinafter called as aSAW filter of a comparative example 3. FIG. 17 is a graph showing aresult of measuring the characteristic as to a SAW filter of thiscomparative example 3. As understood from FIG. 17, in the SAW filter ofthe comparative example 3, losses at 824 MHz and 849 MHz were 0.93 dB,1.29 dB.

Judging from these results, the SAW filter of the practical example 5 ischaracterized on such a point that it can be miniaturized as a whole,although an improvement level is small as compared with the SAW filterof the comparative example 3.

In the SAW filter shown in the above-described practical example,gradation is formed in such a manner that a pitch is changedcontinuously, but the present invention is not limited to this. Forexample, a pitch may be changed in a stepwise fashion. In the SAW filtershown in the above-described practical example in the same manner, shownwas such a configuration that weighting method was applied to IDT, butthere is no limitation to the suchlike configuration and withdrawalweighting method may be applied.

Although it is not explicitly explained, in the first embodiment and theSAW filters of the practical examples manufactured based upon it, thatresonance frequencies of the first IDT and the second IDT are set up tofrequencies necessary for obtaining a preset filter characteristic, butit is figured out from TABLE 1 through TABLE 5 and their explanations,as a matter of course. It is possible to obtain a favorablecharacteristic in the same manner, even if resonant frequency of thefirst IDT is nearly matched with anti-resonance frequency of the secondIDT.

In the present invention, it is all right even if it is configured insuch a manner that a third IDT, which is connected between a signal pathand a ground, is arranged, on an opposite side to such a side that asecond IDT is arranged to a first IDT in such a manner that electrodefingers of its comb-shaped electrode are located continuously, in such amanner that electrode fingers of its comb-shaped electrode are locatedcontinuously to electrode fingers of the comb-shaped electrode of thefirst IDT. Alternately, it is also all right even if it is configured insuch a manner that a fourth IDT, which is connected serially to a signalpath, is arranged, on an opposite side to such a side that a first IDTis arranged to a second IDT in such a manner that electrode fingers ofits comb-shaped electrode are located continuously, in such a mannerthat electrode fingers of its comb-shaped electrode are locatedcontinuously to electrode fingers of the comb-shaped electrode of thesecond IDT. By adopting these configurations, it is possible to realizea SAW filter which is of a small size and a low loss, in the samemanner.

Second Embodiment

FIG. 18 is a plan view showing a configuration of a SAW filter whichrelates to a second embodiment of the present invention. A differentpoint between the SAW filter of this embodiment and the SAW filter ofthe first embodiment is such a point that an L type configuration wasmainly used in the first embodiment, whereas in this embodiment, a πtype configuration is mainly used.

The SAW filter of this embodiment is configured by first IDT 702, secondIDT 703, third IDT 704 on piezoelectric substrate 701, and reflectorelectrodes 709 arranged on both ends.

First IDT 702 is arranged between one terminal 705 of input/outputterminals and other terminal 706 of the input/output terminals, i.e.,serially to a signal path. By this arrangement, this first IDT 702 cancarry out an operation which is equivalent to that of SAW resonators ofa serial arm. Second IDT 703 is arranged in parallel to a signal pathfrom a portion between one terminal 705 of input/output terminals andfirst IDT 702, and connected to first IDT 702 and one terminal 705 bywiring electrode 707. By this arrangement, second IDT 703 can carry outan operation which is equivalent to that of SAW resonators of a parallelarm.

Third IDT 704 is arranged in parallel to a signal path from a portionbetween other terminal 706 of input/output terminals and first IDT 702,and connected to first IDT 702 and other terminal 706 by wiringelectrode 708. By this arrangement, third IDT 704 can carry out anoperation which is equivalent to that of SAW resonators of a parallelarm.

First IDT 702, second IDT 703 and third IDT 704 are arranged inproximity to one another, on the same propagation path of surfaceacoustic waves which are excited by respective SAW resonators. It isconfigured in such a manner that, at this time, surface acoustic waves,which are excited by respective SAW resonators, are not negated eachother. On this account, it is desirable to make them in reversed phases.Reflector electrodes 709 are arranged on respective one end sides ofsecond IDT 703 and third IDT 704, respectively, as shown in the figure.The SAW filter of this embodiment is formed by the above-mentionedconfiguration.

Electrode finger pitches are set up in such a manner that an electrodefinger pitch of first IDT 702 is smaller than electrode finger pitchesof second IDT 703 and third IDT 704 and a filter characteristic based ona design value can be obtained. An electrode finger pitch of reflectorelectrode 709 is set up so as to become an intermediate value of anelectrode finger pitch of first IDT 702, and electrode finger pitches ofsecond IDT 703 and third IDT 704.

By adopting the above-mentioned configuration, first IDT 702, second IDT703 and third IDT 704 in the SAW filter of this embodiment are arrangedin proximity to one another, and surface acoustic waves are closed overon the same propagation path. Therefore, respective SAW resonatorsbecome equivalent to such a matter that a resonator length is lengthenedin substance. In consequence, it is possible to improve a characteristicof SAW resonators, and therefore, it is possible to obtain a band passtype and small loss characteristic, as a SAW filter.

By such a matter that first IDT 702, second IDT 703 and third IDT 704are arranged in proximity to one another and a wiring electrode linelength for connecting can be reduced, it is possible to realizeminiaturization as a SAW filter. As to the suchlike SAW filter, a π typeconfiguration is a basic configuration unit.

As explained above, the SAW filter of this embodiment can realize asmall size and small loss characteristic, by arranging a plurality ofSAW resonators in proximity to one another on the same propagation path.

In this embodiment, a reflector electrode was arranged, but it is allright even if the reflector electrode is not disposed by use of such aconfiguration that it is possible to close over surface acoustic wavesin IDTs sufficiently. It is also all right even if it is configured insuch a manner that roughly several pieces of reflector electrodes orstrip line electrodes are disposed between first IDT 702, second IDT 703and third IDT 704. It is preferable to set up an electrode finger pitchof these reflector electrodes or strip line electrodes to anintermediate value of an electrode finger pitch of first IDT 702, andelectrode finger pitches of second IDT 703 and third IDT 704.

It is all right even if dummy electrodes are arranged on first IDT 702,second IDT 703 and third IDT 704, respectively. By optimizing thisarrangement configuration of dummy electrodes, it is possible to reducea loss much more.

Even in this embodiment, it is possible to realize a small size SAWfilter, over improving a filter characteristic, by disposing a gradationarea as explained in the first embodiment and realizing an apodizedweighting method or thinned-out electrode configuration etc.

Third Embodiment

FIG. 19 is a plan view showing a configuration of a SAW filter whichrelates to a third embodiment of the present invention. A differentpoint between the SAW filter of this embodiment and the SAW filter ofthe first embodiment is that an L type configuration is used in thefirst embodiment, and a T type configuration is used in this embodiment.As shown in FIG. 19, the SAW filter of this embodiment is configured byfirst IDT 802, second IDT 803, fourth IDT 804 on piezoelectric substrate801 and reflector electrodes 808 arranged on both end portions.

First IDT 802 and fourth IDT 804 are arranged between one terminal 805of input/output terminals and other terminal 806 of the input/outputterminals, i.e. serially to a signal path. By this configuration, thesethings can carry out an operation which is equivalent to that of SAWresonators of a serial arm.

Second IDT 803 is arranged in parallel to a signal path from a portionbetween first IDT 802 and fourth IDT 804. This second IDT 803 isconnected to one of comb-shaped electrodes which configure first IDT 802and fourth IDT 804, respectively, by wiring electrode 807. By thisconfiguration, second IDT 803 can carry out an operation which isequivalent to that of SAW resonators of a parallel arm.

First IDT 802, second IDT 803 and fourth IDT 804 are arranged inproximity to one another on the same propagation path of surfaceacoustic waves which are excited by respective resonators. It isconfigured in such a manner that, at this time, surface acoustic waves,which are excited by respective SAW resonators, are not negated eachother. On this account, it is desirable to make them in reversed phases.Reflector electrodes 808 are arranged on an opposite side to adjacentsides of first IDT 802, second IDT 803 and fourth IDT 804.

Electrode finger pitches are set up in such a manner that electrodefinger pitches of first IDT 802 and fourth IDT 804 are smaller than anelectrode finger pitch of second IDT 803 and a filter characteristicbased on a design value can be obtained. An electrode finger pitch ofreflector electrode 808 is set up so as to become an intermediate valueof electrode finger pitches of first IDT 802 and fourth IDT 804, and anelectrode finger pitch of second IDT 803.

By adopting the above-mentioned configuration, first IDT 802, second IDT803 and fourth IDT 804 in the SAW filter of this embodiment are arrangedin proximity to one another, and surface acoustic waves are closed overon the same propagation path, and therefore, respective SAW resonatorsbecome equivalent to such a matter that a resonator length is lengthenedin substance. In consequence, it is possible to realize a SAW filterwhich improves a characteristic of SAW resonators and which has a bandpass type and small loss characteristic. By such a matter that first IDT802, second IDT 803 and fourth IDT 804 are arranged in proximity to oneanother and a wiring electrode line length for connection can bereduced, it is possible to realize miniaturization of a SAW filter. Asto the suchlike SAW filter in this case, a T type configuration is abasic configuration unit.

As explained above, a SAW filter of the present invention can realize asmall size and small loss characteristic, by arranging a plurality ofSAW resonators in proximity to one another on the same propagation path.

Even in this embodiment, it is possible to realize a small size SAWfilter over improving a filter characteristic, by disposing a gradationarea and adopting an apodization or thinned-out electrode configurationetc. as explained in the first embodiment. In this embodiment, reflectorelectrode are arranged, but it is all right even if reflector electrodesare not disposed in particular, by use of such a configuration that itis possible to close over surface acoustic waves in IDTs sufficiently.

It is also all right even if roughly several pieces of strip lineelectrodes are disposed between first IDT 802, second IDT 803 and fourthIDT 804. It is preferable to set up an electrode finger pitch of thesestrip line electrodes to an intermediate value of an electrode fingerpitch of second IDT 803 and electrode finger pitches of first IDT 802and fourth IDT 804.

It is all right even if dummy electrodes are arranged on first IDT 802,second IDT 803 and fourth IDT 804, respectively. By optimizing dummyelectrodes based upon a design value, it is possible to reduce a lossmuch more.

In this embodiment, three IDTs are arranged on the same propagationpath, but it is all right even if they are four or more. Also withregard to connection and arrangement, it is all right even if aplurality of IDTs are arranged in proximity to one another.

In this embodiment, a SAW filter of one stage configuration wasexplained, but it is all right even if this one stage configuration SAWfilter is used as a basic SAW element and they are cascade-connected inmultiple stages to realize a SAW filter of a multiple stageconfiguration. It is also all right even if it is not limited to a Ttype configuration, but it is combined with an L type configuration, a πtype configuration or another configuration such as one terminal pairsurface acoustic wave resonator element.

INDUSTRIAL APPLICABILITY

A SAW filter, which relates to the present invention, is a filter with ahigh attenuation characteristic requiring a lot of SAW resonators, andhas such an advantage that it is possible to reduce a chip size andreduce an insertion loss at the same time, and useful for a filter in acommunication field such as portable telephones or in a video field suchas televisions, and so on.

1-18. (canceled)
 19. A SAW filter comprising a piezoelectric substrate,and at least two inter-digital transducers disposed in proximity to eachother on the same surface acoustic wave propagation path on thepiezoelectric substrate, wherein at least one of the inter-digitaltransducers is a first inter-digital transducer connected serially to asignal path, and at least one is a second inter-digital transducerconnected between the signal path and a ground, wherein the firstinter-digital transducer and the second inter-digital transducer aredifferent in resonance frequency, and the first inter-digital transducerand the second inter-digital transducer are formed by such aconfiguration that electrode fingers of comb-shaped electrodesconfiguring inter-digital transducers are arranged almost continuously,wherein electrode fingers in the first inter-digital transducer, whichare the closest to the second inter-digital transducer, and electrodefingers in the second inter-digital transducer, which are the closest tothe first inter-digital transducer, configure peaks and troughs ofsurface acoustic waves, and comb-shaped electrodes are connected incommon on the side having the electrode fingers of the comb-shapedelectrodes which configure the first inter-digital transducer and thesecond inter-digital transducer, and wherein the first inter-digitaltransducer and the second inter-digital transducer fall in reversedphases each other.
 20. The SAW filter of claim 19, wherein resonancefrequencies of the first inter-digital transducer and the secondinter-digital transducer are set up to frequency necessary for obtaininga preset filter characteristic.
 21. The SAW filter of claim 20, whereinresonance frequency of the first inter-digital transducer is nearlymatched with anti-resonance frequency of the second inter-digitaltransducer.
 22. The SAW filter of claim 19, wherein a reflectorelectrode is disposed on the outermost side of the inter-digitaltransducer including the first inter-digital transducer and the secondinter-digital transducer.
 23. The SAW filter of claim 22, wherein astrip line electrode is disposed between the first inter-digitaltransducer and the second inter-digital transducer, and it is configuredin such a manner that electrode fingers of comb-shaped electrodes whichconfigure the first inter-digital transducer and the secondinter-digital transducer, and electrode fingers which configure thestrip line electrode or the reflector electrode are arranged so as to belocated almost continuously.
 24. The SAW filter of claim 23, wherein apitch of the electrode fingers of the strip line electrode is set up toone between a pitch of the electrode fingers of the first inter-digitaltransducer and a pitch of the electrode fingers of the secondinter-digital transducer.
 25. The SAW filter of claim 19, wherein theinter-digital transducers, which configure the SAW filter, are of aconfiguration including dummy electrodes.
 26. The SAW filter of claim19, wherein a third inter-digital transducer, which is connected betweena signal path and a ground, is arranged in proximity to an opposite sideto such a side that the second inter-digital transducer is arranged inproximity to the first inter-digital transducer.
 27. The SAW filter ofclaim 26, wherein resonance frequency of the third inter-digitaltransducer is different from resonance frequency of the firstinter-digital transducer.
 28. The SAW filter of claim 19, wherein afourth inter-digital transducer, which is connected serially to a signalpath, is arranged in proximity to an opposite side to such a side thatthe first inter-digital transducer is arranged in proximity to thesecond inter-digital transducer.
 29. The SAW filter of claim 28, whereinresonance frequency of the fourth inter-digital transducer is differentfrom resonance frequency of the second inter-digital transducer.
 30. ASAW filter configured in such a manner that the SAW filter of claim 19is used as one SAW element and the elements are connected in multiplestages.
 31. A SAW filter comprising a piezoelectric substrate, and atleast two inter-digital transducers disposed in proximity to each otheron the same surface acoustic wave propagation path on the piezoelectricsubstrate, wherein at least one of the inter-digital transducers is afirst inter-digital transducer connected serially to a signal path, andat least one is a second inter-digital transducer connected between thesignal path and a ground, and the first inter-digital transducer and thesecond inter-digital transducer are different in resonance frequency,and the first inter-digital transducer and the second inter-digitaltransducer are formed by such a configuration that electrode fingers ofcomb-shaped electrodes configuring inter-digital transducers arearranged almost continuously, and a pitch of plural electrode fingers,which are arranged in a boundary area of the first inter-digitaltransducer and the second inter-digital transducer, is differentiatedfrom a pitch of electrode fingers which are arranged in respectivecenter areas.
 32. The SAW filter of claim 31, wherein weighting isapplied to at least one of the inter-digital transducers which configurethe SAW filter.
 33. The SAW filter of claim 32, wherein apodizedweighting method is applied to at least one of the inter-digitaltransducers which configure the SAW filter.
 34. The SAW filter of claim32, wherein withdrawal weighting method is applied to at least one ofthe inter-digital transducers which configure the SAW filter.
 35. TheSAW filter of claim 31, wherein the inter-digital transducers, whichconfigure the SAW filter, are of a configuration including dummyelectrodes.
 36. The SAW filter of claim 31, wherein a thirdinter-digital transducer, which is connected between a signal path and aground, is arranged in proximity to an opposite side to such a side thatthe second inter-digital transducer is arranged in proximity to thefirst inter-digital transducer.
 37. The SAW filter of claim 36, whereinresonance frequency of the third inter-digital transducer is differentfrom resonance frequency of the first inter-digital transducer.
 38. TheSAW filter of claim 31, wherein a fourth inter-digital transducer, whichis connected serially to a signal path, is arranged in proximity to anopposite side to such a side that the first inter-digital transducer isarranged in proximity to the second inter-digital transducer.
 39. TheSAW filter of claim 38, wherein resonance frequency of the fourthinter-digital transducer is different from resonance frequency of thesecond inter-digital transducer.
 40. A SAW filter configured in such amanner that the SAW filter of claim 31 is used as one SAW element andthe elements are connected in multiple stages.